a fly in the ointment
A tiny mistake that makes you wish you never did the thing you were doing in the first place.
My new phone is a fly in the ointment. It was working fine, and now it won't charge.
My girlfriend's new pet is a fly in the ointment. I was about to watch a movie, and now I have to clean up after a chicken.
My mom's new job is a fly in the ointment. She was happy, and now she's crying every day.